Electronic excited states of catechol bound to titanium dioxide nanoparticles were investigated using electroabsorption (Stark effect) spectroscopy. The electronic transition at about 400 nm, characteristic for catechol bound to TiO2 is associated with a change in permanent dipole moment by f . vertical bar Delta mu vertical bar = 15.7 D (where f is the local field correction factor), and a small negative change in the polarizability. Electron transfer distance points to the strong charge-transfer character of this transition. The electroabsorption spectra show also another electronic transition 7000 cm(-1) higher energy, partially masked by the TiO2 absorption. (C) 2009 Elsevier B.V. All rights reserved.